This invention relates to a method of epitaxial growth of a material having one lattice constant and/or thermal coefficient of expansion on a material having another lattice constant and/or thermal coefficient of expansion employing an intermediate or buffer film to provide strain relief and more particularly to the epitaxial growth of cubic III-V, II-VI and I-VII compound semiconductor materials on diamond cubic or zinc blend structure substrate or layer support employing an intermediate buffer film comprising a material with a cubic zinc blend structure having a plastic deformation threshold much less than that of both the substrate support and deposited overlayer.
Lattice mismatch between deposited layers or a deposited material on a substrate and different thermal expansions of various support or substrate materials and deposited overlayers in semiconductor heteroepitaxy provides strains in the deposited material causing stress due to lattice mismatch and thermal expansion mismatch, resulting in induced defect formation, wafer warpage and overlayer cracks. When epitaxial overlayers exceed a critical thickness, misfit and threading dislocations will occur during growth and/or subsequent processing or device operation.
During cooldown after epitaxial growth, some accommodation due to misfit dislocations may occur but at lower or room temperature, only elastic deformation is likely. Thus, strains will occur where different thermal expansions are involved at least in temperature ranges where the overlayers and support do not exhibit plastic deformation capabilities to a sufficient degree. As an example, strains equal to approximately 2.2.times.10.sup.-3 for 2 .mu.m thick GaAs layers on Si(100) wafers are reported. See D. A. Neumann et al, "Structural Properties of GaAs on Si and Ge substrates", Journal of Vacuum Science and Technology, Vol. 4(2), pp. 642-644, March/April 1986. These strains are developed due to the differences of thermal expansion between Si and GaAs and the fixed interface atomic arrangement during cooldown to room temperature after growth and, also, likely due to the lack of plastic deformation of these materials during cooldown and at room temperature.
There is a great deal of work currently directed to growing lattice mismatched materials. One of the most presently popular material combinations under study is the growth of GaAs or other III-V materials on diamond structure substrate supports, such as Si or Ge. In seeking to accommodate lattice mismatch, defects are generated in the subsequently deposited overlayer. These defects, if confined to the interface, are believed to be benign. However, defects which thread into the overlayer material affect the performance and operation of any subsequently formed semiconductor device. Recently, work has been concentrated toward the introduction of an epitaxial buffer film or layer or layers capable of relieving strain without adverse effects in the growth of overlayers. The approach is that strain relief might occur via plastic deformation caused by the movement of dislocations through the buffer film or along its interface with other layers. Further, it is recognized that low elastic stiffness coefficients and low plastic deformation thresholds, of the buffer material could aid to concentrate the strain in the buffer film. In this connection, see, for example, the work of H. Zogg set forth in the article, "Strain Relief in Epitaxial Fluoride Buffer layers For Semiconductor Heteroepitaxy", Applied Physics Letters, Vol. 49(15), pp. 933-935, Oct. 13, 1986. In his work, Zogg uses IIa fluorides, such as, CaF.sub.2, SrF.sub.2 and BaF.sub.2 in MBE on supports such as Si, Ge and GaAs to obtain strain relief. In particular, Zogg found in using a CaF.sub.2 intermediate film (2% mismatch to Si at the growth temperature of 700.degree. C.) improves the quality of the BaF.sub.2 overlayer (mismatch 14% relative to Si) and prevents twinning. The process generally employed in the deposition of such buffer films is to deposit at high temperatures (e.g., 700.degree. C.) followed by a low temperature growth (e.g., 350.degree. C. -450 .degree. C.) of the overlayer. However, crack-free CaF.sub.2 layers thicker than about 20 nm were not possible to obtain using his growth apparatus and procedures.
What is needed is a stable buffer material having better "defect absorbing" properties, i.e., confining misfit dislocations to layer interfaces, but capable of templating the crystalline orientation to the deposited overlayer.
It is an object of this invention to employ an improved intermediate buffer film composed of cubic zinc blend compound structures, which have different deformation properties compared to IIa fluorides, i.e., having low plastic deformation threshold and improved template property compared to IIa fluorides due to the fact that the buffer film shares the same zinc blend structure as the overlayer. Furthermore, such cubic zinc blend materials are more chemically suitable and more compatible, deposition-wise, than IIa fluorides.